1. Field of Invention
The invention is related to a thermal fixing device applied to a device for forming an image by an electrophotographic method.
2. Description of Related Art
The device for forming an image by the electrophotographic method, such as a laser printer, has a thermal fixing device for fixing toner transferred onto a paper. As shown in FIG. 7, the thermal fixing device 61 has a heat roller 62 operating as a heater and a pressure roller 63 and applies heat and pressure to a paper 64 passing between the rollers 62, 63 to melt and fix the toner on the paper 64.
A guide plate 65 for guiding the paper 63 is arranged in the upstream side of the rollers 62, 63 relating to the paper feeding direction. The paper 64 is guided by the guide plate 65 and enters a nip portion 66 where the rollers 62, 63 are in contact and the paper 64 is fed to the downstream side while being heated and pressed.
As shown in FIG. 8, the peripheral surface of the heat roller 62 is formed in a reversed arch crown shape and the peripheral surface of the pressure roller 63 is formed in an arch crown shape. A stretching force, in the width direction of the paper 64, is applied to the paper 64 when entered into the nip portion 66 and the paper 64 so fed is stretched to avoid creasing.
As shown in FIG. 7, the guide surface 65a that is an upper surface of the guide plate 65 is arranged lower than a surface S extending so as to include a tangent common to the rollers 62, 63 at the nip portion 66. The guide surface 65a extends almost parallel to the surface S. A front end of the paper 64 is guided by the guide surface 65a and the paper 64 enters the nip portion 66, however, the paper 64 does not contact the guide surface 65a when the paper 64 is held by the nip portion 66. Because the guide plate 65 is heated to an extent by the heat roller 62, the paper 64 could be preheated when contacting the guide plate 65. However, if the paper 64 does not contact the guide plate 65, the paper 64 is not preheated and the toner cannot be fixed certainly.
If the guide surface 65a is arranged closer to the heat roller 62, the paper 64 contacts the guide surface 65a and the paper 64 can be preheated. However, the following problems occur if the guide surface 65a is arranged too close to the heat roller 62.
At the beginning of the fixing process, the front end of the paper 64 is held by the nip portion 66 and the rear end of the paper 64 is held between a photosensitive drum and a transfer roller. After the paper 64 proceeds and the rear end of the paper 64 is released from the photosensitive drum and the transfer roller, the paper 64 is lifted by the reaction of the release. The toner on the paper 64 that is not fixed contacts the photosensitive drum and the adjacent portions, and the toner is scattered.
It is an object of the invention to provide a thermal fixing device for guiding the paper smoothly.
The thermal fixing device of the invention comprises a heater, a pressure member arranged in contact with the heater, a feeder for feeding a medium and a guide member for guiding the medium fed by the feeder to a contact portion where the heater and the pressure member contact, wherein the guide member is arranged so as to guide the medium in contact with the medium between a first surface and a second surface, and the first surface is a surface including a line that links the contact portion and the feeder and the second surface is a surface including a second line that is perpendicular to a first line that links a curvature center in the contact portion of the heater and a curvature center in the contact portion of the pressure member at a point where the first line crosses the contact portion.
The heater is a heat roller that is heated by a heat source and the pressure member is an elastic pressure roller. In this case, the first surface is defined as a surface extending so as to contact a pressure roller side surface of the heat roller and a surface of the feeder. The second surface is defined as a surface including a tangent common to two circles including a circle having a shaft center of the heat roller as a center and a circle having a shaft center of the pressure roller as a center.
A film of an endless belt that is heated by a heat source can be used for the heater. Because the film is very thin and its temperature is easy to increase, the time until the start of the image forming can be shortened.
The guide member can be kept in a float condition electrically to prevent the developer on the medium from being scattered. The guide member can be earthed via a rectifying member (for example, a Zener diode).
The guide member can be urged in the direction of the heater by an urging member. The urging member may be structured to rotate the guide member around a shaft.
The width of the guide member is set smaller than the width of the fed medium.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.